APS are solid state imagers wherein each pixel contains the typical solid state pixel elements including a photo-sensing means, reset means, a charge to voltage conversion means, and additionally all or part of an amplifier. The photocharge collected within the pixel is converted to a corresponding voltage or current within the pixel as discussed in prior art documents such as "Active Pixel Sensors: Are CCD's Dinosaurs?", SPIE Vol. 1900-08-8194-1133 July 1993, by Eric Fossum. APS devices have been operated in a manner where each line or row of the imager is selected and then read out using a column select signal as discussed by E. Fossum in "Active Pixel Sensors: Are CCD's Dinosaurs?", SPIE Vol. 1900-08-8194-1133 July 1993 and by R. H. Nixon, S. E. Kemeny, C. O. Staller, and E. R. Fossum, in "128.times.128 CMOS Photodiode-type Active Pixel Sensor with On-chip Timing, Control and Signal Chain Electronics". Proceedings of the SPIE vol. 2415, Charge-Coupled Devices and Solid-State Optical Sensors V, paper 34 (1995). The selection of rows and columns within an Active Pixel Sensor is analogous to the selection of words and bits in memory devices. Here, the selection of an entire row would be analogous to selecting a word and the reading out of one of the columns of the Active Pixel Sensor would be analogous to selecting or enabling a single bit line within that word. Conventional prior art photodiode devices teach architectures employing 4 transistor designs, where the 4 transistors (4T) are typically the Transfer, Row Select, Reset, and Source Follower Amplifier transistors. While this architecture provides the advantages of yielding APS devices having the capability to easily perform CDS and provide low readout noise, these 4T pixels suffer from low fill factor. Fill factor is the percentage of pixel area that is devoted to the photosensor. Since these 4 transistors and their associated contact regions and signal busses are placed in each pixel, and since contact regions typically consume a large amount of pixel area due to the required overlap and spacings of various layers, the fill factor for the pixel is reduced because of the large area consumed that could otherwise be used for the photodetector. Connection to each of these components to the appropriate timing signal is done by metal busses that traverse the entire row of pixels. These metal busses are optically opaque and can occlude regions of the photodetector in order to fit them into the pixel pitch. This also reduces the fill factor of the pixel. Decreasing the fill factor reduces the sensitivity and saturation signal of the sensor. This adversely affects the photographic speed and dynamic range of the sensor, performance measures that are critical to obtaining good image quality.
Prior art devices employing 3 transistor (3T) based pixels have a higher fill factor than 4T pixels, but these 3T pixels cannot easily perform CDS. Sensors that perform CDS employing 3 transistor based pixels, typically first read out and store an image frame comprising a reset level for each pixel on the sensor. Next the signal frame is captured and read out. The reset level frame stored in memory must then be subtracted from the signal frame at each pixel to provide a pixel signal level that is referenced to the pixel reset level prior to integration. This requires an extra frame of memory in the imaging system, and an extra step in the digital signal processing chain, thus adversely affecting the speed, size and cost of the system.
A typical prior art Photodiode APS pixel is shown in FIG. 1. The pixel in FIG. 1 is a prior art 4 transistor pixel that comprises: a photodiode (PD), and transfer transistor (TG); floating diffusion (FD); reset transistor with a reset gate (RG); row select transistor with a row select gate, (RSG); a source follower input signal transistor (SIG); a row select signal buss (RSSB); a reset gate signal buss (RGSB), and a transfer gate signal buss (TGSB). 2 adjacent pixels are shown, each containing identical but separate transistors and row control signal busses for RG, TG and RSG. As stated above these 4 transistor pixels provide low readout noise with CDS by inclusion of an extra transistor per pixel. However the area required to implement the 4.sup.th transistor reduces the fill factor of the pixel compared to the 3 transistor pixel.
It should be readily apparent that there remains a need within the art to provide an alternate pixel architecture that has higher fill factor, and the capability to perform CDS without the need to capture and store entire frames of image data.